S. Ostanin

Magnetocrystalline anisotropy and compositional order in Fe0.5Pt0.5: Calculations from an ab initio electronic model

The magnetic properties of FePt samples, potential materials for high density recording media, depend sharply on the degree of chemical ordering produced during their preparation. This has prompted our investigation of the chemical order in both paramagnetic (P) and ferromagnetic (F) phases of a Fe0.5Pt0.5 solid solution and its effect upon the magnetocrystalline anisotropy (MAE) of the latter phase. Our “first-principles” theory uses density functional electronic structure calculations and a mean field treatment of both compositional and magnetic “local moment” fluctuations. We find both phases to be unstable to L10 (CuAu)-type order below temperatures of 1975 K (P) and 1565 K (F) which compares well with that found experimentally (≈1600 K) and our estimate of the Curie temperature as 575 K is also in fair agreement (710 K). For L10 order the calculated MAE is uniaxial with a simple form, υL10c(0,0,1)2 sin2 θ, where the coefficient υL10=9.7×108 erg/cm3, c(0,0,1) is the degree of order (between 0 and 0.5)...

An ab-initio theoretical investigation of the soft-magnetic properties of permalloys

Abstract We study Ni 80 Fe 20 -based permalloys with the relativistic spin-polarized Korringa–Kohn–Rostoker electronic structure method. Treating the compositional disorder with the coherent potential approximation, we investigate how the magnetocrystalline anisotropy, K , and magnetostriction, λ , of Ni-rich Ni–Fe alloys vary with the addition of small amounts of non-magnetic transition metals, Cu and Mo. From our calculations we follow the trends in K and λ and find the compositions of Ni–Fe–Cu and Ni–Fe–Mo where both are near zero. These high permeability compositions of Ni–Fe–Cu and Ni–Fe–Mo match well with those discovered experimentally. We monitor the connection of the magnetic anisotropy with the number of minority spin electrons N ↓ . By raising N ↓ via artificially increasing the band-filling of Ni 80 Fe 20 , we are able to reproduce the key features that underpin the magnetic softening we find in the ternary alloys. The effect of band-filling on the dependence of magnetocrystalline anisotropy on atomic short-range order in Ni 80 Fe 20 is also studied. Our calculations, based on a static concentration wave theory, indicate that the susceptibility of the high permeability of the Ni–Fe–Cu and Ni–Fe–Mo alloys to their annealing conditions is also strongly dependent on the alloys’ compositions. An ideal soft magnet appears from these calculations.